Multiple copies of data can be maintained to provide redundancy in case a data storage subsystem becomes unavailable, or the data becomes damaged or lost. One technique for maintaining redundant copies of data is mirroring, in which data stored in a primary storage subsystem is replicated to a remote mirror system.
One type of mirroring is asynchronous mirroring, in which every write to a logical storage unit (a “logical storage unit” refers to some logical partition of a storage subsystem) can be recorded into a log. Entries of the log are then transmitted to the remote mirror system to recreate the corresponding writes to a mirror copy of the logical storage unit maintained in the remote mirror system.
In some cases, it may be desirable to transfer control of a particular logical storage unit from a first controller to a second controller. An issue associated with the transfer of control of the particular logical storage unit from the first controller to the second controller is that the second controller may start performing writes to the particular logical storage unit, which would have to be mirrored to the mirror copy, prior to the first controller completing the submission of entries of the log associated with the first controller.
To avoid corruption of the mirror copy of the particular logical storage unit kept at the remote mirror system due to out-of-order writes to the mirror copy, a possible solution is to shut down the mirror during the transfer operation from the first controller to the second controller. After the transfer is completed, the asynchronous mirror can be rebuilt from scratch. However, having to recreate the mirror in this manner is processing and time intensive, and can adversely affect performance of a storage subsystem. Moreover, while the mirror is offline, there would be no effective way of knowing what writes have been made to the particular logical storage unit.